Duct-type spacer grid with swirl flow vane for nuclear fuel assembly

ABSTRACT

A duct-type spacer grid for nuclear fuel assemblies is disclosed. In this spacer grid, a plurality of duct-shaped grid elements, individually having an octagonal cell, are closely arranged in parallel and are welded together, thus forming a matrix structure. The grid elements do not pass across the center of the subchannel of the assembly, thus effectively reducing pressure loss. Each of the grid elements is formed as an independent cell, and so they effectively resist against a lateral impact. A plurality of integral type swirl flow vanes, having different heights or same height, axially extend from the top of the grid to be positioned within each subchannel. The swirl flow vanes are bent outwardly, and so they do not contact the fuel rods during an insertion of the fuel rods into the cells. In the spacer grid, the fuel rods are supported within the cells by line contact springs without using any dimple. The spacer grid thus uniformly distributes its spring force on the fuel rods and almost completely prevents damage of the fuel rods due to fretting wear.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates, in general, to a spacer grid usedfor placing and supporting a plurality of nuclear fuel rods within anuclear fuel assembly and, more particularly, to a duct-type spacer gridconsisting of a plurality of duct-shaped grid elements individuallyhaving an octagonal cross-section. The grid is also designed to have aplurality of swirl flow vanes at the top of each grid element.

[0003] 2. Description of the Prior Art

[0004] As shown in FIG. 1, a conventional nuclear fuel assembly 100typically comprises a plurality of spacer grids 110, a bottom nozzle101, a top nozzle 102, a plurality of guide tubes 103, and a pluralityof elongated fuel rods 106.

[0005] In the above fuel assembly 100, the elongated fuel rods 106 areregularly and arranged in parallel to form a structure having a squarecross-section while being placed and supported by the spacer grids 110.Each of the grids 110 is fabricated by assembling a plurality ofintersecting inner strips into an egg-crate pattern. The intersectinginner strips are also welded together at their intersections.

[0006] As best seen in FIGS. 2 and 3, a plurality of inner strips 113intersect each other to form a plurality of four-walled cells 108 havinga square cross-section. In each of the four-walled cells 108, twosprings 114 are provided on the interior of two neighboring walls, whiletwo dimples 115 are provided on the interior of the opposite two walls.The strength of the two dimples 115 is higher than the two springs 114.Both the two springs 114 and the two dimples 115 are used for supportingan elongated fuel rod 106 within each cell 108.

[0007] In the above fuel assembly 100, it is necessary for the springs114 and the dimples 115 to effectively support the fuel rods 106 whilerestricting undesirable movement of the rods 106 even when the assembly100 is impacted by any external force applied in an axial direction A, aradial direction B and/or a rotating direction C. Such an externalforce, applied to the assembly 100, may be caused by the coolant flow,an earthquake or any unexpected external impact. In addition, the gridstructure 111, consisting of the intersecting inner strips 113, has tomaintain the originally designed configuration of the cells 108 evenwhen a lateral impact is applied to a sidewall of the grid 110. In thefuel assembly 100, the spring force of the grid 110 may be graduallyreduced due to neutron irradiation of the assembly 110. The spacer grid110 has to be designed to maintain an effective spring force capable ofcontinuing elastic contact of the springs 114 with a fuel rod 106 untilthe existing rod 106 is changed with a new one.

[0008] In the above fuel assembly 100, the fuel rods 106 may be grown inan axial direction A due to the neutron irradiation, and so the grids110 have to be designed to appropriately support the rods 106 whileallowing such an axial growth of the rods 106. However, when the springforce of the grids 110 undesirably exceeds a reference level, the fuelrods 106 may be prevented from being grown in the axial direction A.This sometimes results in a bending of the fuel rods 106. When the fuelrods 106 are undesirably bent as described above, it is difficult tosecure a subchannel 107 within the fuel assembly 100. This deterioratesthe cooling performance of the assembly 100. FIG. 4 shows a subchannel107, formed by four fuel rods 106. On the other hand, when the springforce of the grid 110 is less than the reference level, the grids 110may fail to effectively place or support the fuel rods 106 within theassembly 100. This finally results in vibration or fretting wear of thefuel rods 106, thus severely damaging the rods 106.

[0009] As well known to those skilled in the art, the power output froma nuclear reactor is partially used as an energy source for causing thecoolant to effectively flow within the reactor core. The amount ofpower, required to cause the coolant flow within the core, is determinedby a hydraulic resistance in the flow paths. In a conventional nuclearfuel assembly 100, the flow paths comprise a main flow path and asub-flow path. When the flow paths are designed having a shape whichdisturbs the coolant flow, a large amount of power has to be consumed tocause the coolant flow. On the other hand, when the flow paths aredesigned having a streamline shape, a small amount of power is needed tocause the coolant flow. It is necessary to make the passages effectivelycause the coolant flow using a small amount of power by reducing thehydraulic resistance.

[0010] A typical spacer grid for nuclear fuel assemblies, used in lightwater reactors, may be referred to U.S. Pat. No. 3,395,077. Anotherconventional spacer grid, having a specifically designed inner strip anda fuel rod support spring, may be referred to U.S. Pat. Nos. 4,426,355,4,726,926, 4,803,043 or 4,888,152.

[0011] In the spacer grid of 4,426,355, the inner strips are corrugatedto form a plurality of wavy dimples at regularly spaced positions. Inthe spacer grid of 4,726,926, a plurality of thin and narrow innerstrips intersect each other prior to being welded together at theirintersections, thus forming a grid structure. After the grid structureis formed by the intersecting inner strips, the strips are appropriatelydeformed to form a plurality of flow paths, springs and dimples. In thespacer grid of 4,803,043, the springs of the inner strips are positionedto be diagonally opposite to each other, thus having an increasedeffective spring length. In the above-mentioned spacer grids, each gridconsists of a plurality of intersecting inner strips. In such a spacergrid having the intersecting inner strips, the inner strips pass acrossthe subchannel having a high flow rate. This type of spacer grid is thusproblematic in that it undesirably results in an increase in pressureloss.

[0012] On the other hand, U.S. Pat. No. 4,888,152 discloses a ring-typespacer grid that comprises a plurality of ducts shaped grid elementsindividually having a square cross-section. In order to form a spacergrid, the grid elements are slitted at appropriate portions and areintersected to each other in a way such that the grid elements form agrid structure arranged in pararell. Such a ring-type spacer grid doesnot pass across the subchannel different from the grids having the innerstrips. However, this ring-type grid is problematic in that the fuelrods are placed and supported by rigid corners of the grid elements,thus being apt to be severely damaged when the fuel rods have vibrated.

[0013] As well known to those skilled in the art, there is a differencebetween the output powers of the fuel rods within a reactor core due toa nonuniform distribution of neutron flux. Therefore, a subchannel,adjacent to a fuel rod having a high thermal power output, may be highlyincreased in enthalpy comparing with the other neighboring subchannels.In accordance with an increase in the power output of the fuel rods,coolant in the subchannel having the high enthalpy rise, may be boiledprior to cooling within the other subchannels. There primarily occurs anucleate boiling and secondarily a film boiling of water within thesubchannel having the high enthalpy rise. When a film boiling occurs, abubble film is formed on a fuel rod surface. Such a bubble filmdecreases heat transfer from the fuel rod surface to the coolant, thusincreasing the temperature of the cladding surface of the fuel rod. Suchan increased temperature of the cladding surface results in a partialthermal stress on the cladding. When the temperature of the cladding isfurther increased, both the cladding may be melted. It is thus necessaryto limitedly operate the reactor core in a way such that any filmboiling does not occur in the subchannels. Such an undesirablephenomenon, caused by film boiling in the subchannel, is a so-called“Departure from Nucleate Boiling(DNB)” in the field. The DNB is affectedby the intervals between fuel rods, system pressure, thermal poweroutput, enthalpy rise and core inlet coolant temperature. In order toallow a nuclear fuel assembly to output a high power while being freefrom such DNB, it is necessary to make a uniform temperaturedistribution of coolant within a nuclear reactor. When such a desireduniform temperature distribution of coolant is accomplished, the coolantis prevented from being partially overheated while maximizing thethermal output. Such a uniform temperature distribution of coolant maybe accomplished by effectively mixing the coolant within each subchannelor between a plurality of subchannels of a fuel assembly. In order tomix the coolant within a fuel assembly as described above, a mixingvane, or an integrated mixing device, may be provided on the top of thestrips of the spacer grid. The above mixing vane structure may bedesigned to cause a cross flow of coolant from a subchannel to aneighboring subchannel. Alternatively, the mixing vane structure may bedesigned to cause a swirling flow of coolant within a subchannel oraround a fuel rod.

[0014] U.S. Pat. No. 4,879,090 discloses a typical vane structure formixing the coolant within a nuclear fuel assembly. Another type ofmixing vane structure for nuclear fuel assemblies may be referred toU.S. Pat. Nos. 5,299,245, 5,110,539 or 5,440,599. On the other hand,U.S. Pat. No. 4,726,926 discloses a specifically designed mixing deviceof the flow deflector type.

[0015] The mixing vane structure of 5,299,245 comprises four swirl flowvanes, which have a blade shape with a slitted end and are arrangedwithin each subchannel. The four vanes of such a blade type are designedto swirl the coolant within a subchannel. However, this vane structureis problematic in that since the vane, arranged in the center of thesubchannel, disturbs the smooth flow of coolant. This increases pressureloss in the fuel assembly. On the other hand, the mixing vane structureof 5,110,539 comprises two swirl flow vanes, which have a blade shapewith a slitted end and are arranged in the center of each subchannel.The two vanes of such a blade type are designed to swirl the coolantwithin the subchannel. However, this vane structure is problematic inthat the vanes may be easily damaged by fuel rod insertion. The mixingvane structure of 5,440,599 comprises two swirl flow vanes, which arelaterally supported and are arranged within each subchannel. The twolaterally supported vanes are designed to move coolant from a subchannelto a neighboring subchannel. However, this vane structure is problematicin that the lateral flow of coolant comes into collision with the mainflow in subchannel, thus being disturbed by the main flow of coolant.This finally deteriorates the coolant mixing effect of the grid. Thevane structure, disclosed in U.S. Pat. No. 4,726,926, comprises aplurality of flow deflector with a bent end. The flow deflectors aredesigned to move the coolant from gap channels, defined by the flow pathbetween fuel rods, to the center of the subchannel. However, this flowdeflector is problematic in that the deflected coolant comes intocollision with coolant from an opposite deflector, thus reducing thecoolant mixing effect caused by the cross flow.

SUMMARY OF THE INVENTION

[0016] Accordingly, the present invention has been made keeping in mindthe above problems occurring in the prior art, and an object of thepresent invention is to provide a duct-type spacer grid for nuclear fuelassemblies, which consists of a plurality of duct-shaped grid elementsindividually having an octagonal cross-section capable of effectivelyresisting against a lateral impact, and which does not pass across asubchannel, thus reducing pressure loss.

[0017] Another object of the present invention is to provide a duct-typespacer grid for nuclear fuel assemblies, which effectively generates aswirl flow of water within subchannels, thus improving the thermalmixing performance of the fuel assembly.

[0018] A further object of the present invention is to provide aduct-type spacer grid for nuclear fuel assemblies, which supports eachelongated fuel rod using line contact springs without using any dimple,thus uniformly distributing the spring force on the spring contact areaof the fuel rod, thus almost completely preventing damage of the fuelrod due to a fretting wear.

[0019] In order to accomplish the above object, the present inventionprovides a duct-type spacer grid for placing and supporting a pluralityof elongated fuel rods within a nuclear fuel assembly, comprising: aplurality of duct-shaped grid having an individual regular polygonalcross-section, the grid elements being closely arranged in parallel andassembled together to form a plurality of main flow paths between them,the main flow paths being used for allowing fuel rod coolant to passthrough and having an individual polygonal cross-section, each of thegrid elements including: a plurality of spring windows formed on aplurality of sidewalls of each polygonal grid element; a surface linespring provided within each of the spring windows while being benttoward the center of each grid element at a central portion thereof,thus elastically supporting an external surface of a fuel rod insertedinto each grid element; and a plurality of swirl flow vanes axiallyextending from a top of each grid element and having different heightsor the same height, each of the vanes being bent twice outwardly fromeach grid element toward the center of an associated subchannel andgenerating a swirl flow of coolant.

[0020] Each of the duct-shaped grid elements forms a main flow paththereby allowing coolant to pass through the main flow paths. Theduct-shaped grid elements are arranged in parallel while forming aregular angle between them. The grid elements are, thereafter, weldedtogether at their upper and lower area of wall and at one or more pointsat each of the upper and lower areas of the wall.

[0021] The line contact spring is bent thoroughly from the sidewall ofeach grid element toward the center of the grid element. The spring alsoforms a contact surface when it is brought into line contact with theexternal surface of the fuel rod. Each of the spring windows forms apassage used for allowing coolant to pass through, and is axially formedon an associated sidewall of each grid element while being parallel tothe axis of the grid element, thus having a longitudinal shape.

[0022] Each of the swirl flow vanes is primarily bent outwardly to forma sub-blade and is secondarily bent outwardly to form a main-blade. Thetwo blades are used for generating a swirl flow of coolant. Thesub-blade is outwardly bent at an acute angle relative to the gridelement, with the main blade being outwardly bent from the inclinedportion of the sub-blade toward the center of an associated main flowpath.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

[0024]FIG. 1 is a perspective view of a conventional nuclear fuelassembly;

[0025]FIG. 2 is a perspective view of a conventional spacer grid usedfor placing and supporting a plurality of elongated fuel rods in thefuel assembly of FIG. 1;

[0026]FIG. 3 is a plan view, showing a plurality of four-walled cellsformed by a plurality of intersecting inner strips of the conventionalspacer grid.

[0027]FIG. 4 is a plan view of a subchannel formed by four fuel rodswithin the conventional spacer grid;

[0028]FIG. 5 is a perspective view of a duct-type spacer grid fornuclear fuel assemblies in accordance with the primary embodiment of thepresent invention;

[0029]FIG. 6 is a front view of the duct-type spacer grid of FIG. 5;

[0030]FIG. 7 is a plan view of the duct-type spacer grid of FIG. 5;

[0031]FIG. 8 is a view, showing a plurality of line contact springs usedfor elastically supporting a fuel rod inserted into a duct-shaped gridelement of the spacer grid of FIG. 5;

[0032]FIG. 9 is a perspective view, showing the upper portion of anoctagonal grid element included in the spacer grid of FIG. 5, with twoswirl flow vanes being provided at the top portion of the grid element;

[0033]FIG. 10 is a plan view, showing the upper portion of the spacergrid of FIG. 5, with a plurality of swirl flow vanes being regularlyarranged on the top of the grid;

[0034]FIG. 11 is a perspective view, showing the upper portion of theoctagonal grid element of FIG. 9, with the two swirl flow vanes prior tobeing bent to a desired configuration;

[0035]FIG. 12 is a perspective view of a duct-type spacer grid fornuclear fuel assemblies in accordance with the second embodiment of thepresent invention;

[0036]FIG. 13 is a perspective view of a duct-type spacer grid fornuclear fuel assemblies in accordance with the third embodiment of thepresent invention; and

[0037]FIG. 14 is a perspective view of a duct-type spacer grid fornuclear fuel assemblies in accordance with the fourth embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038]FIG. 5 is a perspective view of a duct-type spacer grid fornuclear fuel assemblies in accordance with the primary embodiment ofthis invention. As shown in the drawing, the duct-type spacer grid 2 ofthis invention comprises a plurality of duct-shaped grid elements 11,individually provided with both a fuel rod support spring 12 of FIG. 8and a swirl flow vane of FIG. 9.

[0039] The above spacer grid 2 is fabricated by horizontally andarranging in parallel the grid elements 11, each of which has anoctagonal cross-section. In such a case, the grid elements 11 are weldedtogether at the upper and lower area of the wall thereof. In the presentinvention, each of the grid elements 11 may be produced using a tubehaving an octagonal cross-section. Alternatively, each of the gridelements 11 may be made of a thin and narrow strip by forming the stripinto a hollow single structure having an octagonal cross-section.

[0040] When the grid element 11 is made of an octagonal tube, the tubeis machined through a pressing process so as to form a plurality ofspring windows 13, 14, line contact springs 12, and swirl flow vanes 30on the tube. On the other hand, when the grid element 11 is producedusing a thin and narrow strip, the strip is primarily formed into a tubestructure having an octagonal cross-section, thus forming a tube havinga desired size. The tube is, thereafter, machined through a pressingprocess wherein a plurality of spring windows 13, 14, surface contactsprings 12, and swirl flow vanes 30 are formed on the tube in the samemanner as that described for the case of using an octagonal tube. Afterthe pressing process, the tube is subjected to a welding process whereinthe edges are welded and seamed together. A desired grid element 11 isthus completely produced.

[0041] The duct-type spacer grid 2, having a plurality of independentoctagonal cells 8 within the grid elements 11, has an agreeablestructure capable of more effectively resisting against a lateral impactin comparison with a conventional grid structure formed using the innerstrips that intersect each other at right angles at the center of asubchannel 107. The reason why the duct-type grid 2 has such astructural advantage is as follows. That is, when the spacer grid 2 isgeometrically designed to have a plurality of independent octagonalcells 8 as described above, the grid 2 more quickly and effectivelytransfers the lateral impact in every direction than in the case of aconventional strip-type spacer grid. Therefore, when the same lateralimpact is applied to both types of spacer grids, the allowable impactload of the duct-type grid of this invention is remarkably greater thanthat of the conventional strip-type grid.

[0042] As shown in FIGS. 6 and 7, a plurality of longitudinal springwindows, or left- and right-side windows 13 and 14 are formed on thesidewall of each of the grid elements 11 through a pressing process,with a strip-shaped line contact spring 12 being left within each of thewindows 13 and 14 while extending at the center of the window. Thecentral portion of each spring 12 is bent toward the center of the gridelement 11. The spring 12 thus elastically supports an elongated fuelrod 6 at the bulged portion when the fuel rod 6 is inserted into thecell 8 of the grid element 11. Within each grid element 11, four linecontact springs 12 are formed on diametrically opposite four of eightsidewalls. Therefore, the four springs 12 uniformly apply the samespring force to the external surface of a fuel rod 6, inserted into thecell 8, while accomplishing a balance. The spring windows 13 and 14 areused as openings for allowing coolant to pass through so as to moreeffectively cool the fuel rods 6 within the spacer grid 2. A collateralobjective of the windows 13 and 14 is to give additional flexibility tothe springs 12.

[0043]FIG. 8 is a view, showing the operation of the springs 12 whenthey elastically support a fuel rod 6 within a grid element 11 of thespacer grid 2. When the springs 12 support the fuel rod 6 within thegrid element 11, the springs 12 are brought into line contact with theexternal surface of the fuel rod 6. Therefore, the spring 12 isso-called a line contact spring. Since the springs 12 come into linecontact with the fuel rod 6 as described above, the surface contact areaof each spring 6 is increased, while contact pressure is applied fromthe spring 12 to the fuel rod 6. Therefore, it is possible for thespacer grid 2 of this invention to minimize surface damage of the fuelrods 6 due to fretting wear.

[0044]FIG. 9 is a perspective view, showing the top portion of anoctagonal grid element 11 included in the spacer grid of this invention,with two integral type swirl flow vanes 30 being provided at the top ofthe grid element 11. As shown in the drawing, each of the two vanes 30comprises two blade parts: a main blade 31 and a sub-blade 32. Withineach of the grid elements 11, the two vanes 30 are positioned to havedifferent heights. In order to form each swirl flow vane 30 within agrid element 11, an extension part, integrally and axially extendingfrom one sidewall of a grid element 11, is primarily bent toward thecenter of the main flow path 7, thus forming a sub-blade 32. Thereafter,the extension part is secondarily bent at the top of the sub-blade 32toward the center of the main flow path 7, thus forming a main blade 31.

[0045] In the swirl flow vanes 30, each sub-blade 32 provides aninclined surface, at which the main blade 31 starts to extend. Thesub-blade 32 maximizes the size of the main blade 31. The differentheights of the flow vanes 30 within each grid element 11 areaccomplished by the different heights of the sub-blades 32. As thesub-blades 32 have such different heights, the cross-sectioned area ofthe flow path gradually varies, thus reducing the pressure loss causedby the swirl flow vanes 30.

[0046]FIG. 10 is a plan view, showing an arrangement of integral typeswirl flow vanes provided at the top of the duct-type spacer grid 2 ofthis invention. As shown in the drawing, two swirl flow vanes 30 areprovided within each main flow path 7 of the spacer grid 2. Since eachof the vanes 30 is bent outwardly, the vanes 30 are almost completelyfree from being undesirably brought into contact with the fuel rods 6.In addition, the swirling directions of the vanes 30 provided at themain flow paths 7 of the grid 2 are designed as follows. That is, theswirl flow vanes 30, provided at the main flow paths 7 on aperpendicular arrangement, are designed in that their swirlingdirections are opposite to each other. However, the vanes 30, providedat the main flow paths 7 on a diagonal arrangement, are designed to havethe same swirling direction.

[0047]FIG. 11 is a perspective view, showing the two swirl flow vanes 30before they are bent to a desired configuration. As shown in thedrawing, each of the vanes 30 extends from a unit grid element 11 whileforming a triangular plate shape having a specifically curved profileand/or a specifically bent linear profile at both edges. Of course, itshould be understood that each of the vanes 30 may have another shape inplace of the above-mentioned triangular shape and/or another edgeprofile in place of the above-mentioned profiles in accordance with adesired swirl flow.

[0048] The above duct-type spacer grid 2 has the following operationaleffect. That is, the grid element 11 of the spacer grid 2 comprises aduct having an octagonal cross-section, and so the grid element 11 doesnot pass across the center of the subchannel 107, through which coolantflows at a high speed. Therefore, the spacer grid 2 reduces pressureloss caused by the grid elements 11. Each of the grid elements 11 isformed as an independent cell 8 for placing and supporting an elongatedfuel rod 6, thus having an improved resistance against a lateral impactapplied to the sidewall of the grid 2.

[0049] Within each of the main flow paths 7 of the spacer grid 2, fourswirl flow vanes 30 are axially positioned to have different heights,thus reducing pressure loss at the main blades 32 of the vanes 30. Sinceeach of the main blades 32 of the swirl flow vanes 30 is bent outwardlyfrom the cells 8, the main blades 32 are almost completely free frombeing undesirably brought into contact with the fuel rods 6 when thefuel rods 6 are inserted into the cells 8.

[0050]FIG. 12 is a perspective view of a duct-type spacer grid 2 a fornuclear fuel assemblies in accordance with the second embodiment of thisinvention. In the spacer grid 2 a of the second embodiment, theconstruction of both the duct-shaped grid elements 11 and the swirl flowvanes 30 remains the same as that described for the primary embodiment.But, the line contact springs 12 a of the spacer grid 2 a are positionedon the sidewalls around the main flow paths 7 different from the springs12 of the primary embodiment.

[0051] Therefore, the spring windows 13 and 14 are positioned on saidsidewalls around the main flow paths 7 in the second embodiment. Thisstructure finally increases the amount of coolant flowing through thewindows 13 and 14 since a large amount of coolant passes through themain flow paths 7. Therefore, the spacer grid 2 a of this embodimentimproves the cooling effect for the fuel rods 6 within the grid elements11.

[0052]FIG. 13 is a perspective view of a duct-type spacer grid 2 b fornuclear fuel assemblies in accordance with the third embodiment of thisinvention. In the spacer grid 2 b of this embodiment, the constructionof both the duct-shaped grid elements 11 and the swirl flow vanes 30remains the same as that described for the primary embodiment. However,the arrangement of the line contact springs 12 b of this embodiment isaltered as follows. That is, the arrangement of the springs 12 b of theneighboring grid elements 11 is rotated at an angle of 45° one by one.In other words, the arrangement of the springs 12 b in the thirdembodiment is accomplished by alternately using the arrangements of thesprings 12 and 12 a of the primary and second embodiments.

[0053]FIG. 14 is a perspective view of a duct-type spacer grid 10 2 cfor nuclear fuel assemblies in accordance with the fourth embodiment ofthis invention. In the spacer grid 2 c of this embodiment, theconstruction of the duct-shaped grid elements 11, the swirl flow vanes30, the line contact springs 12 and the spring windows 13 and 14 remainsthe same as that described for the primary embodiment. However, thespacer grid 2 c of this embodiment further comprises a plurality ofadditional coolant flow windows 15. The additional windows 15 are formedon the sidewalls between the spring-provided sidewalls of each gridelement 11. This structure increases the amount of coolant flow betweenthe cells 8, thus improving the cooling effect for the fuel rods 6within the grid elements 11.

[0054] As described above, the present invention provides a duct-typespacer grid for nuclear fuel assemblies. The spacer grid of thisinvention consists of a plurality of duct-shaped grid elementsindividually having an octagonal cross-section.

[0055] The grid elements are closely arranged in parallel into a matrixstructure prior to being welded together. In the spacer grid, theduct-shaped grid elements do not pass across the center of thesubchannel 107, through which coolant flows at a high speed. Therefore,the spacer grid of this invention effectively reduces pressure losscaused by the grid elements. Each of the grid elements is formed as anindependent cell effectively resisting against a lateral impact appliedto the sidewall of the grid.

[0056] In the duct-type spacer grid of this invention, two swirl flowvanes are axially positioned to have different heights within eachsubchannel 107. The swirl flow vanes thus reduce pressure loss at theirmain blades. In addition, since each of the main blades of the swirlflow vanes is bent outwardly from the cells, the main blades are almostcompletely free from being undesirably brought into contact with fuelrods when the fuel rods are inserted into the cells.

[0057] Another advantage of this invention resides in that eachelongated fuel rod is supported within a cell by line contact springswithout using any dimple, with the surface contact springs beingpositioned at the same height. The spacer grid of this invention thusuniformly distributes its spring force on the spring contact area ofeach fuel rod, and so it almost completely prevents damage of the fuelrod due to fretting wear.

[0058] Although the preferred embodiments of the present invention havebeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A duct-type spacer grid for placing andsupporting a plurality of elongated fuel rods within a nuclear fuelassembly, comprising: a plurality of duct-shaped grid elementsindividually having a regular polygonal cross-section, said gridelements being closely arranged in parallel and and assembled togetherto form a plurality of main flow paths between them, said main flowpaths being used for allowing coolant to pass through and individuallyhaving a polygonal cross-section, each of said grid elements including:a plurality of spring windows formed on a plurality of sidewalls of eachpolygonal grid element; a line contact spring provided between each ofsaid spring windows while being bent toward the center of each gridelement at a central portion thereof, thus elastically supporting anexternal surface of a fuel rod inserted into each grid element; and aplurality of integral type swirl flow vanes axially extending from thetop of each grid element and having different heights, each of saidvanes being bent twice outwardly from each grid element toward thecenter of an associated main flow path and generating a swirl flow ofcoolant.
 2. The duct-type spacer grid according to claim 1, wherein saidpolygonal cross-section of each of the duct-shaped grid elements is anoctagonal cross-section.
 3. The duct-type spacer grid according to claim2, wherein each of said duct-shaped grid elements forms a main flow paththereof for allowing coolant to pass through the main flow path.
 4. Theduct-type spacer grid according to claim 2, wherein said duct-shapedgrid elements are parallely arranged while forming a regular anglebetween them, said grid elements being welded together at their upperand lower area of the wall and at one or more points at each of saidupper and lower area of the wall.
 5. The duct-type spacer grid accordingto claim 1, wherein said line contact spring is bent thoroughly from thesidewall of each grid element toward the center of the grid element,said spring forming a flat contact surface when it is brought into linecontact with the external surface of the fuel rod.
 6. The duct-typespacer grid according to claim 1, wherein each of said spring windowsforms a flow path used for allowing coolant to pass through.
 7. Theduct-type spacer grid according to claim 1, wherein each of said springwindows is axially formed on an associated sidewall of each grid elementwhile being parallel to the axis of the grid element, thus having alongitudinal shape.
 8. The duct-type spacer grid according to claim 1,wherein each of said swirl flow vanes is integral type.
 9. The duct-typespacer grid according to claim 1, wherein each of said swirl flow vanesis primarily bent outwardly to form a sub-blade and is secondarily bentoutwardly to form a main-blade, said two blades being used forgenerating a swirl flow of coolant.
 10. The duct-type spacer gridaccording to claim 9, wherein said sub-blade is outwardly bent at anacute angle relative to the grid element, with said main blade beingoutwardly bent from an inclined portion of said sub-blade toward thecenter of an associated main flow path.
 11. The duct-type spacer gridaccording to any one of claims 1, 8, 9 and 10, wherein said swirl flowvanes are provided within each of the main flow paths in the form of apair, said vanes being positioned at the same height or at differentheights in an axial direction.